Observation of Multiwavelength Generation and Long-Period Pulsations in a Fiber Laser Passively Mode-Locked by Nonlinear Polarization Rotation

We report on the multiwavelength generation in an Er/Yb-doped fiber ring laser with a cavity length of 301 m. Passive mode locking is implemented by nonlinear polarization rotation. Depending on the adjustment of the polarization controller, the laser operated in a mode-locking regime with emission at one, two, or three wavelengths. At certain orientation angles of the polarization controller, the emission was accompanied by long-period pulsations. It is revealed that the emission at several wavelengths is associated with the nonlinear polarization rotation mechanism, which acts as an intracavity spectral filter and restricts mode competition. Long-period pulsations are formed due to the increase in the intracavity losses caused by a change in the orientation angles of the polarization controller paddle. The experimental results may contribute to the development of pulsed fiber laser sources with long cavities.